It is known that oscillators can be incorporated into laser systems to generate laser beams which comprise a train of laser pulses. More specifically, it is known that pulsed laser beams are useful for ophthalmic laser surgery. For example, U.S. Pat. No. 4,764,930 which issued to Bille et al. for an invention entitled "Multiwavelength Laser Source" and which is assigned to the same assignee as the present invention, discloses a system for use in ophthalmic laser surgery which generates a beam of laser pulses wherein the pulses are of extremely short duration and are generated at an extremely high frequency.
The object in creating a pulsed laser beam which can be effectively used for ophthalmic laser surgery is to quickly and efficiently create a beam of highly concentrated laser light pulses. Importantly, the individual laser light pulses in such a beam should be uniform in intensity (pulse to pulse). Stated differently, it is desirable to have a so-called zero--zero mode pulsed laser beam which does not have the detracting characteristics caused by multiple transverse modes wherein the amplitudes of the pulses vary over time.
In longitudinal diodepumped lasers it is very difficult to focus into a small lasermode because of the relative bad beam quality of the laserdiode.
In high power single strip laserdiodes it is particularly the axis parallel to the strip which has a bad beam quality and therefore makes modematching with a small lasermode in that axis very difficult. Bad modematching (in this case a too big pumpbeam compared to the lasermode in the gain medium in one axis) will result in the creation of higher modes in addition to the transverse electromagnetic (TEM) zero--zero mode (TEM.sub.00 -Mode) in that axis. Thus, heretofore it has been necessary to choose between a quick and easy self-starting generation of a pulsed laser beam with a small gainvolume, and accepting the consequences of the resultant multi-mode condition, or getting a single TEM.sub.00 Mode by increasing the gainvolume but accepting the slower and less reliable self starting condition of the laser pulses.
Multi-mode pulsed laser beams have certain specific characteristics which are relatively easily identifiable. For one, there is a central pulse mode (the zero--zero mode). Additionally, there are linked transverse modes which can exist in addition to the central pulse mode. More specifically, the higher modes are diametrically symmetric to the central pulse and, when there is more than one, all have the tendency to align linearly with the bad pumpdiode axis.
Another characteristic of all multi-mode pulsed laser beams is that due to small frequency differences in the laser light between the transverse modes, the interferometrical addition of these modes result in a beating signal which has a frequency that is about five or six times slower than the frequency of pulses in the laser beam. Further, due to the symmetry of the transverse modes in the multi-mode beam, if one side of the mode is removed from the beam on one side of the central pulse, the other side is also removed from the other side of the central pulse.
In light of the above it is an object of the present invention to provide an oscillator for isolating a transverse electromagnetic (TEM) zero--zero mode pulsed laser beam which can benefit from the relatively quick response time for initial generation of a multi-mode pulsed laser beam. It is also an object of the present invention to provide an oscillator for isolating a transverse electro-magnetic (TEM) zero--zero mode pulsed laser beam which can quickly convert an induced multi-mode laser beam into a zero--zero mode laser beam. Still another object of the present invention is to provide an oscillator for isolating a transverse electromagnetic (TEM) zero--zero mode pulsed laser beam which uses closed loop control to maintain a zero--zero mode pulsed laser beam. It is yet another object of the present invention to provide an oscillator for isolating a transverse electromagnetic (TEM) zero--zero mode pulsed laser beam which is simple to operate, relatively easy to manufacture and comparatively cost effective.